This invention is directed to a fluid storage material, and a method of making a fluid storage material, in which particles are secured to one another and/or to a substrate with a crosslinkable binder composition.
Personal care absorbent products typically include an absorbent layer or an absorbent assembly to absorb and retain liquids, and a number of non-absorbent structural layers and non-absorbent structural components to maintain the absorbent layer in a desired location or to enhance the functionality of the absorbent layer. Each component in the absorbent product serves a specific purpose. As more features and functions are added to the product, the bulk of the product tends to increase.
An absorbent layer with a high absorbent capacity is typically bulkier than an absorbent layer with a lower absorbent capacity. For purposes of discretion and comfort, it is desirable to have as thin an absorbent layer as possible, without sacrificing absorbent capacity. Superabsorbent materials make it possible for absorbent layers to be thin while maintaining a high absorbent capacity, but even garments containing absorbent layers of superabsorbent material may be relatively bulky due to all of the additional features of the garment included to prevent leakage, such as surge layers and additional absorbent material in target areas.
Containment flaps are often included around leg openings to capture any excess fluid around the leg openings, while waist dams may be included around the waist opening to prevent the escape of any excess fluid through the waist opening. Although these additional components may be somewhat absorbent, these components typically do not contain the high absorbency of superabsorbent particles (SAP) because of the difficulty in keeping superabsorbent particles attached to a substrate, particularly in a swollen or wet condition.
Various techniques are known for creating additional absorbency in personal care absorbent articles. For example, it is known to use alkoxysilane-grafted poly(ethylene oxide) as an absorbent coating, thereby creating absorbency on non-absorbent surfaces. However, the resulting absorbent surfaces have less absorbent capacity than SAP.
As another example, it is known to produce an absorbent material by applying a water-softened superabsorbent to a supporting substrate without any additional adhesives. However, use of water alone does not provide for attachment in the wet condition.
As yet another example, it is known to react SAP with a polyhydroxy organic compound, such as glycerol, to form covalent bonds with the SAP. These covalent bonds attach the particles to each other and to a suitable substrate. Formation of covalent bonds with the SAP is expected to create stresses during the swelling process that would either inhibit swelling of the SAP or rupture the membrane coating.
Also, it is known to create individual superabsorbent fibers having high absorbent capacity. Such fibers can be formed by combining a superabsorbent resin with a binder component and adding the combination to a fiber base material in a non-bonded web form. The individual fibers have considerable absorbent capacity.
Furthermore, it is known to create absorbent composites containing fine, hydratable microfibril fibers obtained from cellulose or derivatives capable of swelling in water. These fibers can be used to coat superabsorbent particles. The microfibrilar cellulose fiber coating provides a measure of binding to a supporting sheet, such as a nonwoven fabric. Since the microfibrilar cellulose fibers coat the superabsorbent particles, the fibers tend to inhibit migration of the superabsorbent particles but do not form durable attachments, especially when wet. Conventional adhesive materials used to increase or enhance durability of attachment tend to limit access of liquids to the superabsorbent or create significant constraining forces that limit superabsorbent swelling and therefore ultimate capacity.
U.S. Pat. No. 6,403,857 to Gross et al. describes an absorbent structure including an integral layer of superabsorbent polymer particles. The water-swellable superabsorbent polymer particles are adhered to an absorbent layer using a water-based polymeric binder that is latex bonded and/or thermally bonded. Gross et al. also describe a binder that may be a carboxylic polyelectrolyte in admixture with a crosslinking agent. The crosslinking agent has the property of reacting with carboxylic or carboxylate groups of the polyelectrolyte.
Other recent development efforts have provided coating materials for a variety of uses. For example, U.S. Pat. No. 6,054,523, to Braun et al., describes materials that are formed from organopolysiloxanes containing groups that are capable of condensation, a condensation catalyst, an organopolysiloxane resin, a compound containing a basic nitrogen, and polyvinyl alcohol. The materials are reported to be suitable for use as hydrophobic coatings and for paints and sealing compositions.
Others have reported the production of graft copolymers having silane functional groups that permitted the initiation of cross-linking by exposure to moisture. Prejean (U.S. Pat. No. 5,389,728) describes a melt-processible, moisture-curable graft copolymer that was the reaction product of ethylene, a 1-8 carbon alkyl acrylate or methacrylate, a glycidyl containing monomer such as glycidyl acrylate or methacrylate, onto which has been grafted N-tert-butylaminopropyl trimethoxysilane. The resulting copolymers were reported to be useful as adhesives and for wire and cable coatings.
Furrer et al., in U.S. Pat. No. 5,112,919, reported a moisture-crosslinkable polymer that was produced by blending a thermoplastic base polymer, such as polyethylene, or a copolymer of ethylene, with 1-butene, 1-hexene, 1-octene, or the like; a solid carrier polymer, such as ethylene vinylacetate copolymer (EVA), containing a silane, such as vinyltrimethoxysilane; and a free-radical generator, such as an organic peroxide; and heating the mixture. The copolymers could then be cross-linked by reaction in the presence of water and a catalyst, such as dibutyltin dilaurate, or stannous octoate.
U.S. Pat. No. 4,593,071 to Keough reported moisture cross-linkable ethylene copolymers having pendant silane acryloxy groups. The resultant cross-linked polymers were reported to be especially resistant to moisture and to be useful for extruded coatings around wires and cables. The same group has reported similar moisture curable polymers involving silanes in U.S. Pat. Nos. 5,047,476, 4,767,820, 4,753,993, 4,579,913, 4,575,535, 4,551,504, 4,526,930, 4,493,924, 4,489,029, 4,446,279, 4,440,907, 4,434,272, 4,408,011, 4,369,289, 4,353,997, 4,343,917, 4,328,323, and 4,291,136. Since the cured products of these formulations are reported to be useful for coverings for wire and cable, and for non-conductive coatings for electrical conductors, it would be expected that they are durable coatings for which properties such as water absorbency and biodegradability would be a disadvantage.
Water-swellable polymers have reportedly been produced by cross-linking water soluble polymers, such as poly(ethylene oxide). It is known that poly(alkylene oxides), such as poly(ethylene oxide), can be cross-linked through gamma irradiation. Depending upon the degree of irradiation and the degree of cross-linking, the properties of the cross-linked polymer can range from a water soluble material to a hard solid with no appreciable water absorbency. Materials that are substantially non-water soluble, but still absorbent can be made. However, the use of gamma rays requires expensive equipment and time consuming procedures due to safety concerns, and the degree of cross-linking that is obtained is often difficult to control.
Several references have reported the use of chemical cross-linking groups as a method of avoiding the dangers and costs associated with the use of ionizing radiation. U.S. Pat. No. 3,963,605 to Chu reported a water-swellable, cross-linked poly(alkylene oxide) that was produced by heating a mixture of poly(ethylene oxide) with acrylic acid and a free radical initiator such as acetyl peroxide in a hydrocarbon solvent such as hexane, heptane, or cyclohexane. Another alternative was reported in Canadian Pat. No. 756,190, and involved cross-linking through a di-vinyl monomer in the presence of a free radical catalyst. The use of other cross-linking agents, such as a diacrylate, or methyl-bis-acrylamide with a free radical inhibitor, have also been reported.
Lubricious coatings of cross-linked, hydrophilic polyurethane have been reported by Watson in U.S. Pat. No. 6,020,071. Another polyurethane coating is described by Tedeshchl et al., in EP 0992 252 A2, where a lubricious, drug-accommodating coating is described that is the product of a polyisocyanate; an amine donor, and/or a hydroxyl donor; and an isocyanatosilane adduct having terminal isocyanate groups and an alkoxy silane. A water soluble polymer, such as poly(ethylene oxide), can optionally be present. Cross-linking causes a polyurethane or a polyurea network to form, depending upon whether the isocyanate reacts with the hydroxyl donors or the amine donors. This composition provides lubricious benefits from a particular chemistry, which does not appear to provide high absorbency.
There is a need or desire for a fluid storage material that is thin, durable, and possesses a high absorbent capacity. There is a further need or desire for a method of attaching particles to a substrate such that the particles will remain attached to the substrate even while in a swollen or wet condition without causing a significant decrease in absorbent capacity.
In response to the discussed difficulties and problems encountered in the prior art, a new fluid storage material that provides additional utility to the non-absorbent structural components of personal care absorbent products as carriers of absorbent capacity has been discovered. This capability provides for thinner, more conformable products having greater absorbent capacity. Furthermore, the fluid storage material may provide additional functionality such as odor control, cleansing properties, and skin rejuvenation properties, for example.
The fluid storage material includes particles secured to one another and/or secured to a substrate by a crosslinkable binder composition. Use of the crosslinkable binder provides enhanced attachment of the particles in a swollen or wet condition. Furthermore, the binder does not reduce the effective absorbent capacity of the particles and may contribute an additional absorbent capacity of its own.
The crosslinkable binder composition includes a crosslinkable binder that is sufficiently hydrophilic to provide uninhibited access of aqueous fluids to the particles. The crosslinkable binder may be a soluble binder made up of hydrophilic polymers, a blend of hydrophilic polymers containing hydrophilic agents, and/or a blend of hydrophobic polymers containing hydrophilic agents. For example, the binder may be an alkoxysilane-grafted poly(ethylene oxide). One suitable alkoxysilane is methacryloxypropyl trimethoxy silane. As another example, the binder composition may include acrylic acid copolymers and long chain, hydrophilic acrylate or methacrylate esters, such as poly(ethylene glycol) methacrylate having from 1 to 12 ethylene glycol units. Crosslinking capability is provided by acrylate or methacrylate esters that have a functional group that is capable, upon exposure to water, of forming a silanol functional group that condenses to form a crosslinked polymer. A suitable example of such a methacrylate ester is methacryloxypropyl trimethoxy silane.
The binder in the crosslinkable binder composition suitably has a glass transition temperature below about 30 degrees Celsius, or below about 10 degrees Celsius. The crosslinkable binder composition desirably has a bending modulus lower than the bending modulus of the substrate.
In addition to the crosslinkable binder, the crosslinkable binder composition may also include a solvent that does not substantially swell or adversely affect the particles. Suitably, the solvent provides solubility of the binder, and less than 10% by weight of the solvent is imbibed by the particles. An example of a suitable solvent includes alcohol, such as between about 99.5% and about 50% alcohol by weight, and between about 0.5% and about 50% water by weight. In addition to the binder and the solvent, the binder composition may also include one or more modifying agents, such as plasticizers, colorants, and preservatives.
Alternatively, the binder composition may be heated in a suitable device, such as an extruder, to a flowable condition followed by addition of suitable particles to provide a flowable mixture of binder and particles. The particles may be superabsorbent particles, including, for example, a crosslinked form of sodium polyacrylate, sodium polymethacrylate, polyacrylamide, carboxymethyl cellulose, grafted starch, poly(sodium aspartate), poly(vinyl amine), poly(dimethyldiallyl amine), chitosan salt, and/or poly(ethylene imine). Suitably, the particles have a diameter of between about 50 and about 800 microns, or between about 200 and about 400 microns. For some printing applications, the particles may have a diameter of between about 60 and about 80 microns. The particles and the binder may be present in a ratio of between about 1:4 and about 20:1 on the substrate.
Alternatively, or in addition to the superabsorbent, the particles may include an encapsulated agent. For example, the particles may include encapsulated fragrance agents, cleansing agents, and/or skin rejuvenation agents. As another alternative, the particles may be in a powder form, such as activated carbon or sodium bicarbonate.
The substrate may be a nonwoven web, a woven web, a knitted fabric, a sheet of cellulose tissue, a plastic film, a stranded composite, an elastomer net composite, or any other suitable substrate. Examples of suitable types of plastic film substrates include those made of polypropylene, low density polyethylene, high density polyethylene, linear low density polyethylene, and ultra low density polyethylene.
Alternatively, the substrate may be a release surface. Application of the binder/particle mixture provides, after removal of solvent or cooling of a solvent-free flowable mass, a cohesive film or network composed of particles adhered to each other by the binder composition. The resulting thin, high density, flexible film or network of particles can provide the fluid retention function of an absorbent product.
As a further embodiment, the fluid storage material may include the crosslinkable, absorbent, film-forming binder; particles; and fibers. Suitable fibers include, but are not limited to, cellulose powder which is obtained by grinding birch (or other) hardwood fiber to a smaller particle size powder. Other suitable fibers include other hardwood fibers, both northern and southern, including mercerized southern hardwood fibers, and chemically stiffened southern softwood pulp, as well as superabsorbent fibers.
As mentioned, the fluid storage material can be used to make personal care absorbent articles, thereby providing absorbent capacity to non-absorbent structural layers that typically provide little or no absorbent capacity. These modified structural layers suitably have a thickness of between about 0.2 and about 4 millimeters (mm), when measured at a pressure of 0.05 psi, and an absorbent capacity of between about 0.1 and about 1.8 grams per square centimeter (g/cm2). Examples of articles in which the fluid storage material may be used include diapers, diaper pants, training pants, feminine hygiene products, incontinence products, swimwear garments, and the like.
The fluid storage material of the invention can be made by dispersing particles in the crosslinkable binder solution described above, and applying the particles in solution to a substrate or to a release surface. The combined particles in solution may be applied to the substrate or release surface, either continuously or in a pattern, using any of a variety of application processes, including knife over roll coating, roll coating, spraying, and printing. Examples of suitable printing processes include gravure printing, silk screen, and ink jet printing. After the particles in solution have been applied to the substrate or release surface, crosslinking of the binder is then induced. Crosslinking may be induced by a variety of techniques including thermal initiation, radiation initiation, redox chemical reactions, multivalent metal ions, and moisture. Various types of effective radiation initiation include ultraviolet, microwave, and electron-beam radiation. Moisture initiation may be accomplished through hydrolysis and condensation. Multivalent metal ions can initiate crosslinking by complexation. After inducing crosslinking of the binder, the solvent can be removed from the substrate, either by drying the substrate or using any other effective technique to evaporate the solvent.
Alternatively, the binder composition may be heated in a suitable device, such as an extruder, to a flowable condition followed by addition of the particles to provide a substantially solvent-free, flowable mixture of binder and particles. Upon cooling of the solvent-free flowable mass, a cohesive film or network composed of particles adhered to each other by the binder composition is obtained.
With the foregoing in mind, it is a feature and advantage of the invention to provide a thin, durable, high absorbent capacity fluid storage material, and a method of making such a fluid storage material wherein particles remain intact on the material even while in a swollen or wet condition.